TB Skin Test
- When you have diseases or conditions that weaken your immune system and put you at a greater risk of developing active tuberculosis.
- When you have had close contact with someone who has active TB, or work or live in a high risk environment.
- When you have lived for a period of time in a foreign country where TB may be more common.
- Sometimes as part of an examination prior to starting a new job (such as a healthcare worker).
No sample is required. A small amount of purified protein derivative (PPD) solution is injected just under the first layer of skin of your inner forearm.
The TB skin test is used to determine whether the patient has Mycobacterium tuberculosis within their body. Tuberculosis is an infectious disease caused by an acid fast bacillus (AFB) bacterium named Mycobacterium tuberculosis. It may affect many body organs, but primarily affects the lungs (pulmonary). Tuberculosis, (once called consumption), has been a common cause of death for thousands of years.
TB may cause a latent (inactive) infection or an active, progressive disease. About 90% of the people who become infected with TB manage to control its growth and confine the TB to a few cells in the body. The bacteria in these cells are inactive but still alive. The patient is not sick, they do not have any symptoms, and they are not infectious, but they do have a "latent TB infection." If the patient's immune status is later compromised, the latent TB bacteria may begin to grow again, leading to an active case of tuberculosis disease. This active TB does cause illness in the patient and it can be passed to others through respiratory secretions such as sputum (spit or phlegm) or aerosols released by coughing, sneezing, laughing, or breathing. Common symptoms include night sweats, weight loss and a persistent (>3 weeks) cough with blood.
M. tuberculosis can take a very long time to grow in culture, up to 42 days, therefore tests such as the TB skin test can infer the presence of TB within the body within 48-72 hours. If the test is positive, further testing will be done to look for signs of active tuberculosis. This could involve an X-ray, blood tests or sputum culture.
How is the sample collected for testing?
No sample is required. The test is performed on the patient's skin. A purified protein derivative (PPD) solution that contains TB antigens, but not live bacteria, is used to provoke a hypersensitivity skin reaction (a red raised bump) in those who may have been infected by TB. This reaction occurs because they will have the antibodies against TB which will initiate an immune response once exposed to the PPD resulting in the raised bump.
A healthcare professional will wipe your inner forearm with alcohol and let the skin dry. Using a syringe and a tiny needle, they will inject a small amount (0.1mL) of PPD solution just under the first layer of your skin. When done correctly, the injection forms a small bubble of fluid that looks like a blister. The site should be left uncovered and undisturbed; and perfumes and other cosmetics should not be applied. It will then need to be examined by a healthcare professional at 48 and/or 72 hours after the injection to see if a local skin reaction has occurred.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
How is it used?
The TB skin test is used to screen particular populations who are at risk of TB:
- Those with diseases or conditions that weaken their immune systems, such as those with HIV or AIDS that make them more vulnerable to a TB infection.
- Those who are in confined living conditions such as nursing homes, schools, and correctional facilities
- Healthcare workers and others whose occupations bring them in close contact with those who may have active TB
- Those who have been in close contact with someone who has an active case of TB
- Those who come from or have lived for a period of time in a foreign country where TB may be more common.
The TB skin test is also used sometimes in the evaluation of how well a person’s immune system is working, and as part of a routine examination prior to starting school or a new job. Since mothers can pass TB to their unborn children, pregnant women are sometimes screened, but there are some risks involved that should be discussed with the doctor.
The TB skin test is used to help diagnose latent TB infection before it progresses to active disease. If your doctor suspects that you have active tuberculosis disease other tests, such as chest X-rays and AFB cultures may be used to confirm the diagnosis.
When is it requested?
TB skin tests are requested when the doctor wants to screen their patient for a latent TB infection. They may be done yearly in those that are part of a high-risk group - either because they have a disease that weakens their immune system or because they work or live around others in high risk groups. TB skin tests are not used as a general population screen, but may be performed prior to a person joining an at risk population, such as: going to college, or becoming a teacher or healthcare professional.
Since TB is airborne and passed through respiratory secretions, TB skin tests may be requested when someone has been in close contact with a patient who has an active case of TB (although it is usually about 6 weeks after contact and initial infection before a positive result would emerge), or when you have been in a foreign country where TB may be more common.
TB skin tests should not be done when a person has had a previous positive reaction as they are more likely to have a severe local reaction.
What does the test result mean?
At 48 or 72 hours after the start of the test a healthcare professional will usually interpret your test results by looking at the injection site on your forearm. A positive result will form a red and swollen circle at the site of the injection. The size (diameter) of the swollen raised circle determines whether or not it is significant and likely due to a latent TB infection. The size that is considered positive varies with the health status and age of the individual. Even when infected, children, the elderly, and patients who are severely immune compromised (such as those with AIDS) may have smaller, delayed, or even negative reactions to the TB skin test.
Negative results may mean that you have not been exposed to TB or that it is too early, it takes about 6 weeks after infection before a positive test can be determined. If your doctor wants to confirm a negative result they may repeat the TB skin test.
Positive results may be due to a latent or active TB infection or occasionally due to a false positive. Positive results may also be seen in those who have received a Bacillus Calmette-Guerin (BCG) vaccination. This is a vaccine that is not generally used in the United Kingdom but is often routinely administered in other countries that have a higher incidence of TB or to healthcare professionals who are in close contact with TB patients.
Positive results must be followed up by other tests such as chest X-rays to look for signs of active TB disease. If active TB disease is suspected, AFB cultures may be used to confirm the diagnosis.
Is there anything else I should know?
If you have had a positive TB skin test, usually you should not have another one done. A positive will remain positive and the skin reaction of any subsequent TB skin tests will likely become increasingly severe - to the point that they may require medical attention. Even a negative test may still result in moderate pain, itching, or redness. If the test is not done according to guidelines, however, it will need to be repeated.
Limitations of the test include:
- Variability of interpretation: Reading of the results can be subjective and because of this, the measurement of the diameter of the raised reaction can vary.
- False positives: Can be due to the presence of environmental Mycobacterium species or because patients have been given the BCG vaccine. False positives can also if the wrong antigen is administered or when the test is interpreted incorrectly.
- False negatives: If the patient has been exposed within 6 weeks, they may produce a false negative result. You also may not respond to a TB skin test if you have had a recent viral infection or a "live" vaccine for conditions such as measles, mumps, chickenpox, influenza, or if you have overwhelming tuberculosis, another bacterial infection, or if you have a primary immunodeficiency or are taking immune suppressive drugs such as corticosteroids that weaken the immune response.
- Limited shelf life of the purified protein derivative (PPD): Shelf life varies between manufacturers but generally is 6 months to 1 year if freeze dried in original packaging and less (1-30 days) if reconstituted (made into a liquid form) and stored correctly. If the PPD is used past the shelf life or is stored incorrectly, this can also lead to false negative results.
- Unwillingness of some patients to return for test interpretation.
What about the multiple puncture prong test for TB
This is called the 'tine' test, or 'Heaf' test and is rarely used anymore. It involved the use of a device with multiple prongs/pins that were either dipped into a tuberculin solution and then pricked the skin, or pricked the skin through a drop of tuberculin that had been applied to the surface of the skin. The tine test was not considered as accurate because the amount being delivered could not be controlled. Any positive tine tests had to be followed up with the regular TB/PPD skin test.
Should I get a TB skin test if I am pregnant?
Only under your doctor's supervision, if there is a need to do so. There have not been enough studies done to clearly document the safety of the PPD solution during pregnancy. On the other hand, since TB can be passed from mother to child through the amniotic fluid during pregnancy, if you are at an increased risk of contracting TB your doctor may want you to have a TB skin test done.
Are there any other ways to test for latent TB infection?